Methods for making an MEA with peripheral seal or corresponding MEAs or a fuel cell stack containing such MEAs are rather well known from the prior art, wherein the peripheral seal at the margin of the MEA on the one hand should effectively prevent leakage around the membrane between the electrodes (anode and cathode) arranged on different sides of the membrane and on the other hand be as long-lived as possible under the usually harsh operating conditions in an electrochemical cell (fuel cell). The present invention should be especially suitable for use with so-called “flush-cut” MEAs in which the (gas diffusion) electrodes surrounding the membrane in sandwich fashion are flush with the membrane at the margin. “Flush-cut” MEAs can thus be separated or cut out economically from a large-area and possibly already hot-pressed MEA composite—making possible a roll or sheet production.
A number of aspects are relevant for an effective sealing concept in the peripheral sealing of the MEAs, which are generally arranged between bipolar plates in a fuel cell stack.
In particular, a production method should be provided with which a production of a connection of MEAs with peripheral seal should be made possible in an especially simple and reliably reproducible manner, which are suitable—depending on the materials used—either for use in low-temperature (LT) fuel cells with operating temperatures of less than 100° C. or in high-temperature (HT) fuel cells with operating temperatures of (substantially) more than 100° C. In the case of a use of MEAs with peripheral seal according to the invention in HT fuel cells, it is of special importance that the MEAs and the bipolar plates of a fuel cell stack have a (generally very low) coefficient of thermal expansion, which is significantly different from the (generally higher) coefficient of thermal expansion of a sealing material used to seal the MEA. Especially in the case of using a polymer sealing material, one often resorts to elastomers in the prior art, especially for MEAs for low-temperature fuel cells, which due to their elasticity can balance out the mechanical stresses occurring on account of different coefficients of thermal expansion. Moreover, it is of fundamental importance in the context of the necessary sealing of an MEA that the materials used for a sealing must withstand the harsh conditions in an electrochemical cell for an appropriate lifetime, which especially needs to be taken into account for HT-PEM fuel cells and phosphoric acid fuel cells (PAFC) on account of the usual presence of strong acids in the membrane there.
Thus, e.g., it is known from the prior art that the MEA for sealing purposes is provided with a seal of elastomeric material encircling the MEA at the margin and also overlapping the (gas diffusion) electrodes in a marginal region, as is shown in EP 1 759 434 A1, for example. However, the elastomers preferably used there, insofar as these are even suitable for use in high-temperature fuel cells (such as HT-PEM with phosphoric acid-doped membrane) with an operating temperature range, for example, between 100 and 250° C. (or higher), are either comparatively costly or they have a suboptimal (phosphoric) acid resistance and thus a relatively short lifetime.
Moreover, it is known, for example from WO 2004/015797 A1, how to outfit the MEA of a fuel cell in its marginal region with a polyimide or polyether imide frame enclosing the marginal region of the MEA on both sides by lamination (at temperatures lying below the melting point of polyimide). Such a polyimide frame provides an edge reinforcement for the MEA, but not a true seal, since further seals are absolutely necessary here to seal off the MEA against the bipolar or separator plates of a fuel cell, which makes its fabrication relatively costly—besides the high price for suitable polyimides. Moreover, an increasing embrittlement occurs for such polyimide edge reinforcements partly overlapping the MEA on the top and bottom as their service life increases, which is apparently due to the fact that the coefficient of thermal expansion of polyimide is greater than that of a typical MEA (or the membrane). And finally it is to be noted that this sealing concept is only suitable for MEAs in which the membrane projects to the side beyond the two gas diffusion electrodes, since one can only effectively prevent a leakage between anode and cathode side by direct embedding of the margin region of the membrane in the polyimide frame laminated on the marginal region of the MEA. In other words, the concept described in WO 2004/015797 A1 is not suitable for use on flush-cut MEAs in which the membrane and the gas diffusion electrodes are laterally flush with each other.
In WO 99/04446 A1 further seal arrangements are shown for the MEA of a fuel cell, in which an integral seal encircling the respective MEA is sprayed onto the lateral edge of the MEA from the side, once again making use of an elastomer as the sealing material. However, the lateral spraying of the sealing material is relatively cumbersome. Moreover, this sealing concept as well proves to be suboptimal due to the already mentioned drawbacks for the use of elastomer materials, especially for high-temperature fuel cells.
A rather complicated sealing arrangement, known from U.S. Pat. No. 6,596,427 B1, calls for a double sealing concept for electrochemical cells with an outer seal encapsulating the cell stack on at least one side and separate cell seals surrounding the individual MEAs. The MEA here is separated by means of the cell seal from the outer seal, consisting of a thermoplastic material for example, in order to avoid a direct contact between outer seal and MEA.
Further sealing arrangements are known from U.S. Pat. No. 7,722,978 B2, U.S. Pat. No. 7,914,943 B2, DE 10 2006 004 748 A1, DE 197 03 214 C2 and WO 2011/157377 A2. The sealing materials or arrangements used there are either designed specifically for use in low-temperature fuel cells with an operating temperature <100° C. or are not optimally suited for use in HT-PEM (high-temperature polymer-electrolyte-membrane) fuel cells with operating temperatures of (substantially) >100° C.
Against this background, in the context of the present invention there shall be proposed an effective sealing concept for MEAs which can be used as universally as possible for electrochemical cells (including LT or HT fuel cells) and as economically as possible, and which is also suitable in preferred manner for use with “flush-cut” MEAs.